In microwave applications, especially high frequency Monolithic Microwave Integrated Circuits (MMIC) applications, process variations, gain ripple across bandwidth, and thermal variations in the frequency performance are problems that are typically solved by tuning. For example, in MMIC applications, slight variation in the fabrication processes may cause large gain variations in the resulting RF and microwave devices. Once these devices are combined into functional chains of circuitry, the total variation becomes quite large. For example, in a chain of 10 microwave circuits where each circuit has a gain variation of 2 dB, the resulting chain may have a worst case gain variation of .+-.20 dB. If root sum of squares is used to estimate the gain variation and assuming the stages are independent, the gain variation may still be about .+-.6 dB. These large variations of gain are usually not tolerable and therefore the chain of circuitry must be aligned. RF alignment is time consuming and costly in the manufacturing process, especially in high volume production. If volume production of low cost and very complex microwave hardware is to be a reality, a way of compensating for these process variations without tuning is necessary.
In communication or radar systems for example, where a single carrier or continuous wave (CW) signal is swept in frequency or slowly moved through frequency across several available channels, (i.e., in the case of some cellular communication systems), significant gain ripple may result over a broad bandwidth. Gain ripple is undesirable in these types of systems since this ripple may cause amplitude modulation which may distort the desired modulation waveform. Generally these types of systems contain several stages of amplifiers, filters, mixers, attenuators and other circuitry, all of which have associated interface matching networks and contribute ripple to the final composite performance. In broadband systems, tuning is usually applied at interfaces between these components to reduce the ripple of the system. Accordingly, it is desirable to eliminate or reduce this tuning, especially for high volume production.
Another problem encountered in microwave and RF amplifier design is variation over temperature. Typically microwave amplifiers change about .+-.7 dB over a 50.degree. Celsius temperature range. The actual change however depends on the amplifier design and the intrinsic device characteristics. As with the process variations mentioned above, temperature variation become more severe and harder to correct at higher frequencies such as K-band. Therefore, temperature effects are generally analyzed and compensated for over the frequency range of interest. Typically temperature compensation is done by using thermistors that control the gain of a variable gain amplifier. Thermistors, however, generally have poor reliability and are therefore less desirable in high reliability applications. Accordingly, it is desirable to have an amplifier circuit that provides gain compensation over temperature without the use of thermistors.
For high reliability applications such as MIL-SPEC and space qualified hardware, the effects of aging performance degradation, radiation degradation and other end-of-life effects may be taken into account. When these effects are significant, either performance margins should exist or the degregated performance should be compensated for in some fashion. Accordingly, what is also needed is a way of compensating for reduced performance over the life of an amplifier.
Accordingly, what is needed is a microwave circuit that compensates for process variations in the fabrication. What is also needed is a microwave circuit that eliminates tuning or reduces tuning. What is also needed is a microwave circuit that provides flat gain over a broad bandwidth for CW signals. And, what is also needed is a microwave circuit that compensates for performance and temperature variation and end of life variation.